1. Field of the Invention
The present invention relates to a liquid crystal display device used by a single person or a plurality of people, such as a wordprocessor, a personal computer, or a mobile information terminal, a polymerizable compound suitable for such a liquid crystal display device, a polymerizable resin composition containing such a compound, and a cured polymer formed from such a composition. In this specification, a liquid crystal display device will be referred to as an xe2x80x9cLCD devicexe2x80x9d.
2. Description of the Related Art
Conventionally, the following LCD devices using a composite of a liquid crystal material and a polymer are known.
Japanese National Phase PCT Laid-Open Publication No. 58-501631, for example, describes a polymer dispersed liquid crystal display device (hereinafter, referred to as a xe2x80x9cPDLC display devicexe2x80x9d). The PDLC display device includes liquid crystal domains each surrounded by a polymer matrix. When no voltage is applied to the liquid crystal material, the refractive indices of the liquid crystal material and the polymer are not matched, resulting in a scattered state. When a voltage is applied to the liquid crystal material, the refractive index of the liquid crystal material changes so as to be matched with that of the polymer, resulting in a transparent state.
Japanese National Phase PCT Laid-Open Publication No. 61-502128, for example, describes an LCD device in which a mixture of the liquid crystal material and a photopolymerizable resin is irradiated with ultraviolet light, resulting in three-dimensional phase separation into a liquid crystal material and a polymer.
The above-described LCD devices perform display by electrically controlling the liquid crystal material so that it is in the scattered state or the transparent state.
Japanese Laid-Open Publication No. 1-269922 describes the following method. A mixture of a liquid crystal material and a photopolymerizable resin is subjected to a first exposure to ultraviolet light through a photomask. After the photomask is removed, the mixture is subjected to a second exposure with ultraviolet light, so that an area covered by the photomask during the first exposure is irradiated with ultraviolet light. Thus, areas having different display characteristics are formed. An LCD device produced by this method basically performs in a scattered mode.
Japanese Laid-Open Publication No. 5-257135 describes an LCD device produced by the following method. An alignment layer having an alignment restricting force is provided on each of two substrates. Into a gap between the two substrates, a mixture of a liquid crystal material and a photopolymerizable resin is injected. The mixture of the liquid crystal material and the photopolymerizable resin is irradiated with ultraviolet light through a photomask. The LCD device produced by this method is used for static driving, by which liquid crystal domains are patterned by a control performed outside the cell, utilizing that an area covered by the photomask and an area not covered by the photomask have different threshold characteristics.
As an attempt to improve the viewing angle characteristic of the LCD devices, use of a composite of a liquid crystal material and a polymer has been proposed. It is necessary that liquid crystal molecules are oriented in at least three directions in a pixel area in order to improve the orientation state of the liquid crystal molecules and thus the viewing angle characteristic.
With reference to FIGS. 8A and 8B, the viewing angle characteristic of a wide viewing angle mode LCD device are described.
FIG. 8A schematically shows the relationship between a change in the orientation of liquid crystal molecules and the viewing angle characteristic in accordance with application of a voltage regarding a wide viewing angle mode LCD device 10. FIG. 8B schematically shows such relationship regarding a conventional twisted nematic(TN) mode LCD device. In both FIGS. 8A and 8B, part (a) illustrates the state where no voltage is applied, part (b) illustrates an intermediate state where a voltage is applied to a certain degree less than full, and part (c) illustrates the state where the voltage is applied to a full degree.
As shown in FIG. 8A, the wide viewing angle mode LCD device 10 includes substrates 1 and 2. A liquid crystal layer interposed between the substrates 1 and 2 includes a polymer wall 7 surrounding a liquid crystal domain 8. Liquid crystal molecules 9 included in the liquid crystal domain 8 are oriented symmetrically with respect to an axis 6. Accordingly, in the intermediate state shown in part (b), the apparent refractive indices of the liquid crystal molecules 9 when seen in directions A and B are averaged to be equal. As a result, the viewing angle characteristic is improved compared to the TN mode shown in FIG. 8B.
In the conventional TN mode LCD device shown in FIG. 8B, the liquid crystal molecules have only one orientation direction in the intermediate state shown in part (b). Accordingly, display characteristics such as the levels of brightness and the apparent refractive indices of the liquid crystal molecules when seen in directions A and B are different. As a result, the viewing angle characteristic is inferior to that of the LCD device 10.
The following LCD devices are disclosed as wide viewing angle mode LCD devices.
Japanese Laid-Open Publication Nos. 4-338923 and 4-212928 disclose a wide viewing angle mode LCD apparatus produced by combining the above-described PDLC display device and polarizers having polarizing axes perpendicular to each other.
Japanese Laid-Open Publication. No. 5-27242 discloses a method for improving the viewing angle characteristic of a non-scattering mode LCD device using polarizers. According to the method, a mixture of a liquid crystal material and a photopolymerizable resin is phase-separated, thereby forming a liquid crystal layer formed of a composite of the liquid crystal material and a polymer. Due to such a method, the orientation of liquid crystal domains becomes random by the resultant polymer. In other words, the liquid crystal molecules in different domains rise in different orientation directions when a voltage is applied. As a result, the transmittance of the liquid crystal molecules becomes equal when seen in a plurality of directions, thus improving the viewing angle characteristic in a half-tone display.
The applicant of the present invention discloses the following LCD device in Japanese Laid-Open Publication No. 6-301015. The LCD device is produced by controlling the amount of light using a photomask or the like during photopolymerization, so that the liquid crystal molecules are oriented in an all-direction state (axially symmetrical orientation) in a pixel area, and that the area which is not covered with the photomask forms a polymer wall mainly composed of a photocurable resin.
In the above-described LCD device, disclination is generated at the interface between the polymer wall and the liquid crystal domain due to a reverse tilt of the liquid crystal molecules in the liquid crystal domain. Such disclination is exhibited as a bright line on the display and thus the display characteristics degrade in a black state.
In order to overcome the problem of generation of disclination when a voltage is applied, the applicant of the present invention discloses, in Japanese Laid-Open Publication No. 7-120728, a technique where a polymerizable compound having a liquid crystal-like structure is added to a mixture of a liquid crystal composition and a photocurable resin.
By using the above polymerizable compound, however, the following two problems arise.
First, the pretilt of the liquid crystal molecules in the liquid crystal domain becomes large in a normally-white mode, reducing the brightness of the display when no voltage is applied.
Secondly, the response speed, the threshold characteristic and sharpness in the voltage vs. transmittance characteristic, and the like decrease due to the interaction between the polymer and the liquid crystal material in the composite layer and between the polymer wall and the liquid crystal domain at the interface thereof, and the like.
In the LCD device having an improved viewing angle characteristic, such as the device disclosed in Japanese Laid-Open Publication No. 6-301015, it is difficult to (1) control the orientation of liquid crystal molecules so that the liquid crystal molecules are oriented omnidirectionally, and (2) prevent a reduction of contrast caused by depolarization of light which is scattered at the interface of the liquid crystal material and the polymer.
In order to suppress the light scattering at the interface of the liquid crystal material and the polymer, the occurrence of such an interface in a pixel area may be reduced. In order to reduce the occurrence of the interface, the size and the position of liquid crystal droplets produced in a three-dimensional polymer matrix must be controlled. In conventional methods, however, such control is extremely difficult.
In order to solve the above problems, it is important that the above polymerizable compound used for suppressing generation of disclination should be suitably selected so that the response speed and the voltage vs. transmittance characteristic can be prevented from decreasing. It would be advantageous, also, if at least one liquid crystal droplet can be formed in one pixel area.
In other words, in order to realize an LCD device where a conventional liquid crystal mode is pseudo-solidified, the above problems of controlling the orientation of liquid crystal molecules and controlling the scattering intensity at the interface must be solved preferably simultaneously or at least individually. It is therefore very important to obtain a polymerizable compound which can solve both of these problems.
In the case of using a polymerizable compound having a liquid crystal-like structure described above, if such a compound has a comparatively high polymerization, it is likely to disturb the orientation of liquid crystal molecules and provide an excessively strong memory effect at the interface of the liquid crystal material and the polymer, causing a printing afterimage phenomenon and the like. Providing a suitable polymerizable compound is therefore a critical problem.
The polymerizable compound of this invention is represented by general formula (I): 
where R is H, Rxe2x80x2, Rxe2x80x2O, Rxe2x80x2COO, or Rxe2x80x2OCO, Rxe2x80x2 is a linear or branched alkyl group or alkenyl group having 1 to about 15 carbon atoms; A1 and A2 are independently a cyclohexane ring or a benzene ring which may include a substituent represented by formula (II) below; X is H or 3; and Y1, Y2, Y3, and Y4 are independently H, F, Cl, CH3, CH3O, CF3, or CF3O wherein at least two of Y1, Y2, Y3, and Y4 are H and, if both A1 and A2 are cyclohexane rings, at least one of Y1, Y2, Y3, and Y4 is not H: 
where Y5, Y6, Y7, and Y8 are independently H, F, Cl, CH3, CH3O, CF3, or CF3O, at least two of Y5, Y6, Y7, and Y8 are H.
In one embodiment of the invention, in general formula (I) A1 is a cyclohexane ring and A2 is a benzene ring.
In another embodiment of the invention, in general formula (I) both A1 and A2 are cyclohexane rings.
Alternatively, the polymerizable compound of this invention is represented by general formula (III): 
where R is H, F, or a linear or branched alkyl group or alkoxy group having 1 to 15 carbon atoms of which an arbitrary hydrogen atom may be substituted by a fluorine atom; Y1, Y2, Y3, Y4, Y5, Y6, Y7, and Y8 are independently H or F; X is H or CH3 wherein at least one of Y1, Y2, Y3, Y4, Y5, Y6, Y7, and Y8 is F if R is a linear alkyl group of which an arbitrary hydrogen atom is not substituted by a fluorine atom.
According to one aspect of the invention, a polymerizable resin composition is provided, which includes a polymerizable resin material containing the polymerizable compound described above and a photoinitiator mixed with each other.
According to another aspect of the invention, a cured polymer is provided, which is obtained by polymerizing the polymerizable resin composition described above.
According to still another aspect of the invention, a liquid crystal display device is provided. The liquid crystal display device includes a polymer wall and liquid crystal domains surrounded by the polymer wall, the polymer wall and the liquid crystal domains being interposed between a pair of substrates, wherein the polymer wall includes the cured polymer described above.
In one embodiment of the invention, the polymerizable resin composition includes the polymerizable compound in an amount equal to or more than about 3 wt. % and equal to or less than about 40 wt. %.
Alternatively, the liquid crystal display device of this invention includes a polymer wall and liquid crystal domains surrounded by the polymer wall, the polymer wall and the liquid crystal domains being interposed between a pair of substrates, wherein at least part of an area of the polymer wall which is in contact with the liquid crystal domains includes the cured polymer described above.
In one embodiment of the invention, liquid crystal molecules in each of the liquid crystal domains are allowed to be oriented in an axially symmetric state.
In another embodiment of the invention, the liquid crystal domains are arranged regularly.
In still another embodiment of the invention, the liquid crystal display device further includes a liquid crystal alignment layer provided on a surface of at least one of the pair of substrates, the surface facing the liquid crystal domains.
In still another embodiment of the invention, liquid crystal molecules in the liquid crystal domains are oriented in one of a twisted nematic manner, a super twisted nematic manner, an electrically controlled birefringence manner, and a surface stabilized ferroelectric liquid crystal manner.
In still another embodiment of the invention, the liquid crystal domains are each provided for one pixel area which is a minimum unit for display.
Arbitrary hydrogen means one or more hydrogen atoms on the alkyl or alkoxy group is randomly substituted by a fluorine atom.
Thus, the polymerizable compound according to the present invention is a compound having a liquid crystal-like structure and a polymerizable functional group in molecules. The existence of a liquid crystal-like structure in molecules serves to stabilize the orientation of liquid crystal molecules in liquid crystal domains. The polymerizable functional group is a styrene or -methylstyrene group which has an excellent selectivity during the polymerization reaction and a high polymerization rate. This allows for a reduction of the anchoring strength of the liquid crystal molecules by adjusting the interaction between the liquid crystal molecules and the polymer at the interface thereof.
Thus, the invention described herein makes possible the advantages of providing (1) a liquid crystal display device which does not disturb the orientation state of liquid crystal molecules, minimizes a decrease of the response speed and the voltage vs. transmittance characteristics, has an improved contrast ratio, and suppresses an occurrence of a printing afterimage phenomenon even in a fixed display, (2) a polymerizable compound suitable for such a liquid crystal display device, (3) a polymerizable resin composition containing such a compound, and (4) a cured polymer formed from such a composition.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.